1. Field of the Invention
The present invention relates to an improved recoverable and reusable die used for cold forging. The die provides a high number of uses for cold forging under a normal temperature. The plastic film on the frame of the die is taken off when changing the shape of the die cavity. An alloy with a low melting point is used so that the die cavity can be softened and a new plastic film can be used to form a die cavity with a new shape. The die is re-solidified to be suitable for cold forging. In this way, a die can be recovered and reused.
2. Description of the Prior Art
The basic structure of a die for cold forging as disclosed in the present invention is described in U.S. Pat. No. 5,727,420. The structure includes a die frame formed from metallic material, a bottom lid with bolt holes, a suitable number of strut spacers welded on the bottom lid, a plastic film provided on top of the die frame, an upper filler layer and a lower filler layer lapped with each other in the die frame, and a suitable amount of steel beads (or iron pellets). The plastic film is shaped to conform to the shape of a die cavity formed by a vacuum forming technique using PET or PVC plastic material. The upper filler layer is formed by overlapping a plurality of sheets made of material such as Kevlar. The lower filler layer is formed by several sheets of stainless steel mesh.
In manufacturing the die, the upper and the lower filler layers are placed below the plastic film in the die frame. Then liquid mercury is poured into the die as a medium of penetration in the basic die structure. The liquid mercury penetrates the gaps formed by the mesh of the stainless steel nets and small holes on the sheets of Kevlar. After a fast freezing operation, the liquid mercury in the die is frozen to solidify the basic die structure and becomes a solidifying medium in the basic die structure.
The prior art takes advantage of the physical characteristic of mercury which is solidified at -38.5.degree. C., so that the liquid mercury is poured into the die as a medium of penetration in the basic die structure under a normal temperature, and the die is fast frozen to -40.degree. C. with chilling equipment. The mercury thus expands during solidification to support the basic die structure.
However, in the stated prior art, expensive mercury is chosen as a medium of penetration in the basic die structure. Although mercury provides the advantage of repeated and recoverable use of the fillers for the die, higher cost is required for manufacturing the die. The temperature of the die must be lowered to be below -40.degree. C., which requires expensive chilling equipment.